append shader⊶node group from another blender file - in update style
AFAIK There is no direct link back to the node group in your template file once the group is appended so what you would have to do is appending the new group and replace the data-block of the old one:

You might want to use python to automate this process for all materials within the blend file:
import bpy

D = bpy.data

for mat in D.materials:
    if hasattr(mat.node_tree, &quot;nodes&quot;):
        for node in mat.node_tree.nodes:
            if node.type == 'GROUP':
                if node.node_tree.name == &quot;Template v1&quot;:
                    node.node_tree = D.node_groups['Template v2']